1. Field of the Invention
The present invention relates to a wiring substrate for high frequency applications that has a transmission line structure for transmitting therethrough a high frequency signal such as a microwave signal, millimeter wave signal, or the like, and more particularly to a wiring substrate for high frequency applications that improves reflection characteristics for high frequency signals at transmission line interconnects between the upper and lower surfaces of a dielectric substrate.
2. Description of the Related Art
In a high frequency wiring substrate that has a transmission line structure for transmitting a high frequency signal therethrough, interconnects between transmission line conductors formed on the upper and lower surfaces of a dielectric substrate have traditionally been achieved using through-hole conductors such as plated via holes, as shown in the perspective view of FIG. 5.
In the perspective view of FIG. 5, the interior of the high frequency wiring substrate is shown by dotted lines. As shown in FIG. 5, surface transmission line conductors 2 and 3 as transmission lines for high frequency signals are formed in line with each other on the upper and lower surfaces of the dielectric substrate 1 consisting of a single or multiple dielectric layers, and the surface transmission line conductors 2 and 3 are connected together at their opposing ends by a signal conducting through-hole conductor 4 such as a plated via hole. On the upper and lower surfaces of the dielectric substrate 1 are also formed surface ground conductors 5 and 6 in such a manner as to surround the respective surface transmission line conductors 2 and 3. The surface transmission line conductors 2, 3 and the surface ground conductors 5, 6 together constitute grounding coplanar transmission lines on the upper and lower surfaces of the dielectric substrate 1, respectively. Further, the surface ground conductors 5 and 6 are connected together by a large number of through-hole ground conductors 7 formed in substantially parallel arrays on both sides of the respective surface transmission line conductors 2 and 3. This structure not only achieves stable grounding of the grounding coplanar transmission lines formed on the upper and lower surfaces of the dielectric substrate 1, but also serves as an impedance matching grounding structure in the area surrounding the signal conducting through-hole conductor 4.
However, in the high frequency wiring substrate of the prior art shown in FIG. 5, as the frequency of the high frequency signal to be transmitted increases, the length of the signal conducting through-hole conductor 4 becomes closer to or nearly the same as the guide wavelength at the signal frequency in the dielectric substrate 1, resulting in the problem that the degree of impedance mismatch increases, increasing the reflection of the high frequency signal; there has also been the problem that since the thickness of the dielectric layer also becomes close to the guide wavelength at the signal frequency in the dielectric layer, unwanted mode conversion to parallel plate mode or waveguide mode occurs within the dielectric layer, degrading the transmission characteristics.
In view of increasing frequency of high frequency signals to be transmitted, it is desired to realize a transmission line structure that can transmit high frequency signals between transmission line conductors on the upper and lower surfaces of the dielectric substrate without degrading the transmission characteristics.
The present invention has been devised in view of the above problems of the prior art, and an object of the invention is to provide a wiring substrate for high frequency applications that has a transmission line interconnect structure capable of efficiently transmitting signals in high frequency regions including microwave and millimeter wave regions, by improving the reflection characteristics of high frequency signals at transmission line interconnects between the upper and lower surfaces of a dielectric substrate and suppressing the occurrence of conversion to an unwanted mode within the dielectric substrate.
The present invention provides a wiring substrate for high frequency applications, comprising: a pair of surface transmission line conductors formed in line with each other on upper and lower surfaces of a dielectric substrate composed of a stack of dielectric layers, each of the surface transmission line conductors being surrounded by a surface ground conductor; and a transmission line interconnect structure for transmitting a high frequency signal between the pair of surface transmission line conductors via an interlayer transmission line conductor formed between the dielectric layers in such a manner as to be in line with and parallel to the pair of surface transmission lines, the interlayer transmission line conductor being surrounded by an interlayer ground conductor and connected at both ends thereof to the respective ends of the pair of surface transmission line conductors by signal conducting through-hole conductors, wherein in the transmission line interconnect structure, the surface ground conductors on the upper and lower surfaces are connected together via the interlayer ground conductor by top-to-bottom through-hole ground conductors arranged in two parallel arrays spaced a prescribed distance apart from each other on both sides of the interlayer transmission line conductor, while the interlayer ground conductor is connected to the surface ground conductors on the upper and lower surfaces by interlayer through-hole ground conductors arranged in an array at a prescribed pitch along each of two sides extending perpendicularly to the interlayer transmission line conductor; and the thickness of each of the dielectric layers, representing the spacing between the interlayer transmission line conductor and the respective surface transmission line conductors, and the length of a straight line section of the interlayer transmission line conductor between the signal conducting through-hole conductors are each set not larger than one quarter of a signal wavelength of the high frequency signal, while through-hole ground conductors for connecting the surface ground conductors on the upper and lower surfaces are formed in a region where the interlayer ground conductor is not formed between the interlayer transmission line conductor and the top-to-bottom through-hole ground conductors arranged parallelly on both sides of the interlayer transmission line conductor, and are arranged at a pitch not larger than a prescribed pitch that has the frequency of the high frequency signal as a cutoff frequency.
Furthermore, the invention also provides a wiring substrate for high frequency applications, comprising:
(a) a dielectric substrate composed of a stack of dielectric layers;
(b) a first surface transmission line conductor formed in a straight line on one surface of the dielectric substrate;
(c) a second surface transmission line conductor formed in a straight line on the other surface of the dielectric substrate;
(d) a first surface ground conductor formed to surround the first surface transmission line conductor;
(e) a second surface ground conductor formed to surround the second surface transmission line conductor; and
(f) a transmission line interconnect structure for transmitting a high frequency signal between the first surface transmission line conductor and the second surface transmission line conductor, the transmission line interconnect structure comprising:
(f1) an interlayer transmission line conductor formed in a straight line between the dielectric layers;
(f2) an interlayer ground conductor formed between the dielectric layers in such a manner as to surround the interlayer transmission line conductor;
(f3) signal conducting through-hole conductors for connecting the first surface transmission line conductor to the interlayer transmission line conductor, and for connecting the second surface transmission line conductor to the interlayer transmission line conductor;
(f4) first through-hole ground conductors, arranged in two parallel arrays spaced a prescribed distance apart from each other on both side of the interlayer transmission line conductor, for connecting the first surface ground conductor to the second surface ground conductor via the interlayer ground conductor;
(f5) second through-hole ground conductors for connecting the first surface ground conductor to the interlayer ground conductor, and for connecting the second surface ground conductor to the interlayer ground conductor, the second through-hole ground conductors being arranged in an array at a prescribed pitch along each of two sides extending perpendicularly to the interlayer transmission line conductor; and
(f6) third through-hole ground conductors for connecting the first surface ground conductor to the second surface ground conductor, the third through-hole ground conductors being formed in a region where the interlayer ground conductor is not formed between the interlayer transmission line conductor and the first through-hole ground conductors arranged parallelly on both sides of the interlayer transmission line conductor,
wherein a thickness of each of the dielectric layers, representing the spacing between the interlayer transmission line conductor and the first and second surface transmission line conductors, and a length of a straight line section of the interlayer transmission line conductor between the signal conducting through-hole conductors are each set not larger than one quarter of a signal wavelength of the high frequency signal, and
the third through-hole ground conductors are arranged at a pitch not larger than a prescribed pitch that has the frequency of the high frequency signal as a cutoff frequency.
According to the high frequency wiring substrate of the present invention, since the surface transmission line conductors on the upper and lower surfaces of the dielectric substrate are connected together using the above-described transmission line interconnect structure, the signal conducting through-hole conductors provided at both ends of the interlayer transmission line conductor can each be held shorter than the guide wavelength in the dielectric substrate. Further, since the interlayer ground conductor, which serves as a ground plane in the high frequency transmission line structure having the upper (lower) surface transmission line conductor as the signal conductor, is connected by the interlayer through-hole ground conductors to the lower (upper) surface ground conductor which serves as a ground plane in the high frequency transmission line structure having the interlayer transmission line conductor as the signal conductor, ground discontinuities where the structure changes are eliminated, and therefore, the increase in reflections due to the signal conducting through-hole conductors and the interlayer transmission line conductor connecting between the upper and lower surface transmission line conductors can be held low. Generally, between discontinued points along a transmission line, there can exist a standing wave such that the spacing between the discontinued points becomes equal to an integral multiple of a half wavelength; however, in the present invention, since the straight line section of the interlayer transmission line conductor between the signal conducting through-hole conductors is set not longer than one quarter of the guide wavelength of the high frequency signal, the wavelength of the standing wave occurring between the discontinued points at the signal conducting through-hole conductors connected to both ends of the interlayer transmission line conductor can be shifted to a frequency region higher than the signal frequency. Further, in a rectangular waveguide structure, the cutoff frequency increases as the length of the longer side of the cross section decreases; in view of this, the through-hole ground conductors for connecting the upper and lower surface ground conductors, which are formed in the region where the interlayer ground conductor is not formed between the interlayer transmission line conductor and the top-to-bottom through-hole ground conductors arranged parallelly on both sides of the interlayer transmission line conductor, are arranged at a pitch not larger than the prescribed pitch that has the frequency of the high frequency signal as the cutoff frequency. As a result, the cutoff frequency of the dielectric waveguide transmission line structure can also be shifted to a frequency region higher than the signal frequency, the dielectric waveguide transmission line structure here comprising: the top-to-bottom through-hole conductors arranged in two parallel arrays spaced a prescribed distance apart from each other on both sides of the interlayer transmission line conductor and extending along the signal transmission direction thereof in such a manner as to surround the interlayer transmission line conductor; the upper and lower surface ground conductors; the interlayer transmission line conductor; and the through-hole ground conductors for connecting the upper and lower surface ground conductors, which are formed in the region where the interlayer ground conductor is not formed between the interlayer transmission line conductor and the top-to-bottom through-hole ground conductors arranged parallelly on both sides of the interlayer transmission line conductor. This improves the reflection characteristics of high frequency signals at the transmission line interconnects between the upper and lower surfaces of the dielectric substrate and suppresses the occurrence of conversion to an unwanted mode within the dielectric substrate; as a result, the high frequency wiring substrate thus constructed has the transmission line interconnect structure capable of efficiently transmitting signals in high frequency regions including microwave and millimeter wave regions.
As described above, according to the high frequency substrate of the present invention, which comprises a pair of surface transmission line conductors formed in line with each other on the upper and lower surfaces of a dielectric substrate, each surface transmission line conductor being surrounded by a surface ground conductor, and a transmission line interconnect structure for transmitting a high frequency signal between the surface transmission line conductors via an interlayer transmission line conductor formed between the dielectric layers in such a manner as to be in line with and parallel to the surface transmission lines, the interlayer transmission line conductor being surrounded by an interlayer ground conductor and connected at both ends thereof to the respective ends of the surface transmission line conductors by signal conducting through-hole conductors, the transmission line interconnect structure is characterized in that: the surface ground conductors on the upper and lower surfaces are connected together via the interlayer ground conductor by top-to-bottom through-hole ground conductors arranged in two parallel arrays spaced a prescribed distance apart from each other on both sides of the interlayer transmission line conductor, while the interlayer ground conductor is connected to the surface ground conductors on the upper and lower surfaces by interlayer through-hole ground conductors arranged in an array at a prescribed pitch along each of two sides extending perpendicularly to the interlayer transmission line conductor; and the thickness of each of the dielectric layers, representing the spacing between the interlayer transmission line conductor and the respective surface transmission line conductors, and the length of the straight line section of the interlayer transmission line conductor between the signal conducting through-hole conductors are each set not larger than one quarter of the signal wavelength of the high frequency signal, while through-hole ground conductors for connecting the surface ground conductors on the upper and lower surfaces are formed in a region where the interlayer ground conductor is not formed between the interlayer transmission line conductor and the top-to-bottom through-hole ground conductors arranged parallelly on both sides of the interlayer transmission line conductor, and are arranged at a pitch not larger than a prescribed pitch that has the frequency of the high frequency signal as the cutoff frequency. With the structure, the increase in the inductance component associated with the signal conducting through-hole conductors and the interlayer transmission line conductor connecting between the upper and lower surface transmission line conductors can be held low. Furthermore, the wavelength of the standing wave occurring between the discontinued points at the signal conducting through-hole conductors connected to both ends of the interlayer transmission line conductor can be shifted to a frequency region higher than the signal frequency. At the same time, the cutoff frequency of the dielectric waveguide transmission line structure formed in such a manner as to surround the interlayer transmission line conductor can also be shifted to a frequency region higher than the signal frequency. This improves the reflection characteristics of high frequency signals at the transmission line interconnects between the upper and lower surfaces of the dielectric substrate and suppresses the occurrence of conversion to an unwanted mode within the dielectric substrate; as a result, the high frequency wiring substrate thus constructed has the transmission line interconnect structure capable of efficiently transmitting signals in high frequency regions including microwave and millimeter wave regions.
In the invention it is preferable that the distance between the two arrays of top-to-bottom through-hole ground conductors is set about twice the spacing between the pair of surface transmission line conductors.